The present invention relates to a three phase control system, in detail, to a three phase load controlling system having two bilateral switching elements connected in an arbitrary two of three phases between the load and the power supply, a trigger circuit for applying gating signals to the two bilateral switching elements and a synchronizing circuit for providing electrical signals in synchronism with the frequency of the power supply, for the trigger circuit to provide a predetermined time base from which the retard angles of the trigger circuits can be referenced, whereby the current from the power supply to the load is controlled in only two phases.
Conventionally the trigger signals of switching elements must be synchronized to the line-to-neutral voltages in order to achieve 180.degree. control with a resistive load. However, for short conduction angles a large single phase current can circulate through the load.
To prevent this disadvantage, a gating system as is disclosed in U.S. Pat. No. 3,497,796 has been developed. According to this system, the first switching element is gated on by a trigger circuit which is synchronized by a synchronizing signal which leads the line-to-neutral voltage of the first phase by an angle of 30.degree. and the second switching element is gated on by a trigger circuit which is synchronized by a synchronizing signal which lags the line-to-neutral voltage of the third phase by an angle of 30.degree..
As will be understood from the description in column 3, lines 19 to 26 of the specification of this patent, however, this gating system results in an inability to achieve 180.degree. control of the current in the third phase. With a minimum retard angle, the line-to-neutral voltage of the third phase is 30.degree. past the zero crossover and this portion of the supply voltage cannot be gated on.
In case the load is an induction motor which drives an elevator cage, the above mentioned inability becomes a limitation in the range of speed control. If the induction motor rotates at constant speed for the entire duration of its operation, the load power factor may remain at a constant degree, e.g. 30.degree., lagging. However, the induction motor which drives the elevator cage must be controlled over a wide range of changes in speed and load. The load power factor cannot be constant, for example at 30.degree. lagging.